Materials Chemistry and Physics 113 (2009) 1003–1008
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Organic light-emitting devices based on solution-processible
quinolato-complex supramolecules
Jung-An Cheng a,∗ , Chin H. Chen b , Han-Ping D. Shieh a
a
b
Department of Photonics & Display Institute, National Chiao Tung University, Hsinchu 30010, Taiwan
Microelectronics and Information System Research Center, National Chiao Tung University, Hsinchu 30010, Taiwan
a r t i c l e
i n f o
Article history:
Received 22 October 2007
Received in revised form 27 August 2008
Accepted 29 August 2008
Keywords:
OLEDs
Solution processable
Alq3
EL
a b s t r a c t
This paper discusses a new type of supramolecular material tris{5-N-[3-(9H-carbazol-9-yl)propyl]-N(4-methylphenyl)aminesulfonyl-8-hydroxyquinolato} aluminum(III), Al(SCarq)3 , which we synthesized
using three 5-N-[3-(9H-carbazol-9-yl)propyl]-N-(4-methylphenyl)aminesulfonyl-8-hydroxyquinoline as
bidentate ligands. The peak photoluminescence in the solid phase appears at 488 nm. In cyclic voltammetric measurement, two oxidation peaks, which were obtained at −5.6 and −5.9 eV, correspond to HOMO
sites of carbazoyl and aluminum quinolates, respectively. In the investigation of solid morphological thin
film, the flat surface was investigated using an atomic force microscope. The root mean square (rms) and
mean roughness (Ra ) were respectively measured to be 0.427 and 0.343 nm. For the fabrication of organic
light-emitting devices (OLEDs) using spin-coating techniques, the turn-on voltage and maximum luminescence of the optimized electroluminescence device, glass/ITO (20 nm)/PEDOT:PSS (75 nm)/Al(SCarq)3
(85 nm)/BCP (8 nm)/LiF (1 nm)/Al (200 nm), were respectively 9.6 V and 35.0 cd m−2 . Due to the electroplex formation between the carbazole (electron-donor) and the aluminum quinolates (electron-acceptor)
moieties under an applied DC bias, the chromaticity of electroluminescence shifted to green-yellow with
1931 CIEx,y (0.40, 0.47).
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Since 1987, Tang and VanSlyke discovered the first high
efficiency organic light-emitting devices (OLEDs) based on tris(8quinolinolato)aluminum (Alq3 ) [1]. Most small molecules-based
OLEDs were fabricated using thermal evaporation techniques.
Because of the relative ease of synthesis and purification, small
molecules of guest-host dopant systems based on Alq3 are particularly suitable for fabrication of full color OLEDs [2]. On the
contrary, solution processed small molecule OLEDs are rare except
for a few relatively large frameworks of dendriatic molecules [3,4],
dye dispersed polymers [5], and oligomers [6]. In pushing the
small molecule OLED fabrication technology forward large area
substrates, there exists a need to simplify the manufacturing process and to reduce the cost of these highly efficient doped devices.
Development of OLEDs fabrication technology via solution processible small molecule materials offers a simple alternative and also
∗ Corresponding author at: Department of Photonics & Display Institute, National
Chiao Tung University, Room 502 CPT Building, 1001 Ta Hsueh Road, Hsinchu 30010,
Taiwan. Tel.: +886 3 5712121x59210; fax: +886 3 5737681.
E-mail address: [email protected] (J.-A. Cheng).
0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2008.08.084
opens up exciting possibilities for inkjet printing fabrication similar
to what has been demonstrated in PLEDs [7].
Although there are several materials utilized in device fabrication using spin-coating, such as demdrimers [4] and light-emitting
polymers [8], little research has been done on a solution-process
based on six-coordinated aluminum quinolate complexes without
a polymeric backbone or matrix [9–12].
In the development of OLEDs, a wide band-gap of host materials is required for sensitizing various dopants. According to
the semi-empirical approach of the Zerner International Neglect
of Differential Overlap (ZINDO) theory [13], the band-gap of
the quinolato aluminum complexes is tunable by introducing an
electron-withdrawing group at C5 in 8-hydroxyquinoline. The corresponding derivates with sulfonamide substitutent were also
demonstrated by Hopkins et al. [14]. However, these complexes
cannot be sublimated because they are limited in thermal process. To overcome this problem and promote its applications in
OLEDs, we have derived a novel solution-processible host material,
meridional tris(5-N-ethylanilinesulfonamide-8-quinolato-N1 ,O8 )
aluminum, Al(Saq)3 . Electroluminescence (EL) was demonstrated
by the solution process. However, due to the absence of holetransporting ability, its electroluminescent was observed to be
noticeably suppressed [15].
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J.-A. Cheng et al. / Materials Chemistry and Physics 113 (2009) 1003–1008
In this study, based on Al(Saq)3 , we attempt to promote the
hole-transporting ability and EL performance in devices by introducing a well-known hole-transporting moiety, carbazole, into
the bidentate quinoline to produce a novel and functionalized
tris[5-N-(3-carbazylpropyl)-p-methylaniline-sulfonyl-8-hydroxyquinolato] aluminum(III)—hereby abbreviated as Al(SCarq)3 . All
properties of Al(SCarq)3 are characterized and discussed.
2.3. Photoluminescence (PL)
For solution investigation, the compound was dissolved in 1,2-dichloroethane
(c = 10−3 –10−4 mol L−1 ). For solid-phase investigations of pure compounds, thin films
of 1,2-dichloroethane solution (1.0 wt.%) were spin-coated. UV–vis absorption spectra were measured with an HP spectrophotometer by using either the solutions in
cuvettes of 10 mm path length or the solid thin film coated on quartz plates. PL
spectra were recorded with an Acton Research Spectra Pro-150 spectrometer. For PL
experiments, the excitation wavelength was set at the absorption maximum of the
complex.
2. Experimental
2.4. Device fabrication
2.1. General comments
All chemicals used in this research are commercially available. Tetrahydrofurane
(THF) was distillated from Na/benzophenone. Al(Saq)3 was prepared according to
our previous work [15]. 1 H NMR was recorded with a Varian Unity-300 Hz spectrometer or AS-500 MHz NMR spectrometer. Elemental analyses on the samples were
obtained using a HERAEUS CHN-OS RAPID. The thermal properties were determined
by means of differential scanning calorimetry (DSC, Seiko SSC 5200).
2.2. Synthesis
2.2.1. N-[3-(9H-carbazole-9-yl)propyl]-N-(4-methylphenyl)amine (2)
In a 250-mL round-bottom flask fitted with a magnetic stirrer, condenser,
and N2 inlet were placed p-methylaniline (2.97 g, 27.76 mmol), 9-(3-bromopropyl)9H-carbazole (compound 1 [16], 5.00 g, 17.35 mmol), calcium carbonate (1.74 g,
17.40 mmol), and acetonitrile (100 mL) with stirring. The reactants were refluxed
for 36 h and then cooled at room temperature. The solid formed was filtered off and
then the filtrate was concentrated under reduced pressure with heating at 100 ◦ C to
remove the solvent. Thereafter, the residual crude product was flashed through silica
gel (eluent: hexane/ethyl acetate = 6/1) to produce 2 (4.49 g, yield 82.5%). 1 H NMR
(300 MHz, CDCl3 ) ı 1.927–2.00 (m, 2H), 2.16 (s, 3H), 3.13 (t, 2H), 3.60 (bs, 1H), 4.20
(t, 2H), 6.43 (d, J = 8.4 Hz, 2H), 6.86 (d, J = 8.4 Hz, 2H), 7.13–7.24 (m, 4H), 7.33–7.39 (m,
2H), 8.04 (d, J = 7.7 Hz, 2H). FT-IR (KBr) 3052, 2717, 2847, 1597, 1516, 1497, 1479, 1451,
1362, 1335, 1315, 1230, 788, 749, 724, 698 cm−1 . MS-EI (m/e, rel. intensity) 314 (M+ ,
97.7), 315 (M+ +1, 17.02). Anal. Calcd for C22 H22 N2 : C, 84.04; H, 7.05; N, 8.91. Found:
C, 84.31; H, 7.44; N, 8.64.
2.2.2. 5-N-[3-(9H-carbazol-9-yl)propyl]-N-(4-methylphenyl)aminesulfonyl
-8-hydroxyquinoline (3)
In a 100-mL round-bottom flask fitted with a magnetic stirrer, a condenser,
and a N2 inlet were placed 8-hydroxyquinoline-5-sulfonyl chloride [17] (2.00 g,
8.21 mmol) with 20 mL of dichloromethane. Triethylamine (3.41 mL), compound 2
(3.09 g, 9.84 mmol), and dichloromethane (10 mL) mixtures were added slowly into
the reactor. The reaction mixture was heated to reflux for 24 h. The solution was then
filtered, and the solid byproduct was removed. Solvent was stripped off by rotary
vapor, the solid was crystallized in ethyl acetate/methanol (1:1), and white crystals
were collected to yield 3.33 g (65.0%). Mp 144–145 ◦ C. 1 H NMR (300 MHz, CDCl3 ) ı
2.02–2.10 (m, 2H), 2.33 (s, 3H), 3.66 (t, J = 6.6 Hz, 2H), 4.35 (t, J = 7.5 Hz, 2H), 6.48 (d,
J = 8.7 Hz, 2H), 6.91 (d, J = 8.4 Hz, 2H), 7.18–7.30 (m, 6H), 7.37–7.44 (m, 2H), 7.99–8.11
(m, 3H), 8.37 (d, J = 11.1 Hz, 1H), 8.75 (d, J = 4.4 Hz, 1H). FT-IR (KBr) 3359 (br), 3041,
2936, 2878, 1744, 1594, 1518, 1500, 1483, 1452, 1403, 1372, 1342, 1326, 1225, 1202,
1190, 1152, 1130, 1065, 749, 721, 682 cm−1 . MS-EI (m/e): 521 (M+ ), 522 (M+ +1). Anal.
Calcd for C31 H27 N3 O3 S: C, 71.38; H, 5.22; N, 8.06; O, 9.20; S, 6.15. Found: C, 71.12; H,
5.04; N, 8.44.
2.2.3. Tris{5-N-[3-(9H-carbazol-9-yl)propyl]-N-(4-methylphenyl)amine
-sulfonyl-8-hydroxyl-quinolato} aluminum(III) (4)
A 200-mL flask fitted with a N2 inlet, a septum, and a stir bar was flame-dried
and cooled under N2 . Compound 3 (1.50 g, 2.88 mmol) was added along with anhydrous THF (60 mL). A hexane solution of triethylaluminum (1.1 mL, 15, w/w%) was
added via syringe. A yellow solution with blue-green fluorescence was immediately
visible. The reaction was stirred at room temperature for 48 h. The crude product was recrystallized in THF to afford yellow solid (1.39 g, yield 91.1%) of 4. Mp
219–220 ◦ C. Tg = 154.1 ◦ C. 1 H NMR (300 MHz, CDCl3 ) ı 1.94–2.08 (m, 6H), 2.29 (s,
3H), 2.34 (s, 3H), 2.380 (s, 3H), 3.54–3.60 (m, 2H), 3.64–3.68 (m, 2H), 3.74–3.76 (m,
2H), 4.26 (t, J = 4.2 Hz, 2H), 4.31 (t, J = 4.5 Hz, 2H), 4.36 (t, J = 4.8 Hz, 2H), 6.90–6.94
(q, 6H), 6.97–7.08 (m, 13H), 7.23–7.17 (m, 8H), 7.35–7.43 (m, 9H), 8.04–8.11 (m,
9H), 8.44 (d, J = 5.4 Hz, 1H), 8.51–8.56 (m, 3H), 8.65 (d, J = 2.7 Hz, 1H), 8.69 (d,
J = 2.1 Hz, 1H). FT-IR (KBr) 3049, 2924, 2850, 1599, 1576, 1498, 1484, 1463, 1494,
1373, 1332, 1233, 1201, 1166, 1153, 1136, 816, 750, 723, 701, 657, 545, 425 cm−1 . The
molecular weight was characterized by Bruker Biflex III with the matrix-assisted
laser desorption/ionization (MALDI) [18] method which revealed no extraneous
peak other than the desired parent m/e = 1589.8. Anal. Calcd for C93 H78 AlN9 O9 S3 :
C, 70.30; H, 4.95; N, 7.93; O, 9.06; S, 6.05; Al, 1.70. Found: C, 70.28; H, 4.72;
N, 7.55.
The two different device structures, ITO/PEDOT:PSS/Al(SCarq)3 /LiF/Al and
ITO/PEDOT:PSS/Al(SCarq)3 /BCP/LiF/Al, were prepared for EL measurement in
the following manner. For all structures, the indium-tin-oxide (ITO, sheet
resistance is 20 −1 ) glass substrates were cleaned ultrasonically with
detergent, de-ionized water, iso-propanol and methanol. After cleaning, poly(3,4ethylenedioxythiophene)/poly(styrene)-sulfonate (PEDOT:PSS, obtained from
Bayer) aqueous solution was spin-coated at 3000 rpm onto the cleaned ITO
substrates and baked at 110 ◦ C for 24 h. The Neat Al(SCarq)3 were subsequently
spin-coated on the PEDOT layer at 2500 rpm to give the film typically 85 nm thick.
The 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, 8 nm), lithium fluoride
(LiF, 1 nm) and Al layer (200 nm) were sequentially deposited at 1 × 10−6 Torr. The
emitting area of the device was 3 mm × 3 mm. The voltage–current–luminance
characteristics were measured using an optical power meter (PR-650) and a digital
source meter (Keithley 2400). The EL spectra were carried out in an ambient
atmosphere after encapsulation.
3. Result and discussion
3.1. Synthesis and characterization of Al(SCarq)3 complex
The chemical structure and the synthetic procedure of the
supramolecular Al(SCarq)3 are illustrated in Scheme 1. Via SN 2
reaction, carbazole, and p-methylaniline were connected with
a propyl spacer to enable N-[3-(9H-carbazole-9-yl)propyl]N-(4-methylphenyl)amine (2) which reacted further with 8hydroxyquinoline-5-sulfonyl chloride to obtain 5-N-[3-(9Hcarbazol-9-yl)propyl]-N-(4-methylphenyl)aminesulfonyl-8hydroxyquinoline (3). Finally, reacting ligand 3 with triethylaluminum produced the desired six-coordinated Al(SCarq)3 (4).
This functionalized supramolecule has good solubility in a variety
of common organic solvent, such as 1,2-dichloroethane, THF, and
dimethyl sulfoxide (DMSO). In our study, it was also found that the
solubility of this organometallic complex significantly improved
when R was replaced instead of H by methyl group (–CH3 ) as
shown in Scheme 1.
3.2. Cyclic voltagemeteric measurement
To realize the highest occupied molecular orbitals (HOMOs)
energy levels, the oxidation electrochemical behavior of Al(SCarq)3
was examined by using cyclic voltammetry (CV) on Pt electrodes in
1,2-dicholorethane. Here, (Bu)4 NBF4 and Ag/AgCl (saturated) were
utilized as the supporting electrolyte and the reference electrode,
respectively. The potential values were reported with respect to
the ferrocene standard. The functionalized Al(SCarq)3 showed three
stepwise-oxidation waves, which peaked at 1.54, 1.02 and 0.96 V, as
shown in Fig. 1. Evidently, the inserted propyl spacer cannot only
separate the conjugated system between the core and carbazoyl,
but also keeps their individual redox potential constant. Therefore, two HOMOs energy levels were measured using CV, and their
corresponding values, carbazoyl and core, were −5.6 and −5.9 eV,
respectively.
3.3. Net thin films of Al(SCarq)3 formed by solution-process
High quality thin film formation is essential for the potential use
of solution-processible materials in manufacturing organic devices.
J.-A. Cheng et al. / Materials Chemistry and Physics 113 (2009) 1003–1008
1005
Scheme 1. Synthesis of the Alq3 -based supramolecule. (I) p-Methylaniline/K2 CO3 /acetonitril/reflux, (II) 8-hydroxyquinoline-5-sulfonylchloride/CH2 Cl2 /triethyl amine, and
(III) triethylaluminum/THF.
Fig. 2 shows the AFM image of the solid thin film morphology coated
with 1.0 wt.% Al(SCarq)3 in 1,2-dichloroethane. The AFM image
reveals that the film is flat and pin-hole-free. Its root mean square
(rms) and mean roughness (Ra ) are 0.427 and 0.343 nm, respectively. The thin film phase prepared by spin-coating is presumably
possible for most small molecular materials due to the formation of
intermolecular hydrogen bonding or physically entanglement. The
better Al(SCarq)3 thin film obtained by solution process could be
due to two factors. First, the intermolecular entanglement occurred
from the packing of sulfonamide groups at C-5 of the quinolate
ring. This was also demonstrated by Cheng et al. [15] and Qiao
et al. [19]. Second, it could be the magnitude of the molecular weight, Mw = 1589.8 g mol−1 , which increased since carbazoles
were introduced into the Al(SCarq)3 . Thus, the viscosity of the solution increased. As a result, high quality thin-films can be obtained
at lower concentration than that of Al(Saq)3 .
3.4. Absorption and photoluminescence
Fig. 1. Cyclic voltammogram of Al(SCarq)3 (1 mM) complex in 1,2-dichloroethane
with (Bu)4 NBF4 (0.1 M) as the supporting electrolyte.
For comparison and characterization, the optical spectra of
Al(SCarq)3 and Al(Saq)3 are plotted in Fig. 3. As shown in Fig. 3a, the
spectrum of Al(SCarq)3 exhibits characteristic absorption bands of
carbazole groups and the core of aluminum quinolate at 332, 346,
and 376 nm, respectively. In the normalized PL spectra, as shown
in Fig. 3c and e, features of Al(Saq)3 and Al(SCarq)3 are almost the
same in diluted solution, and their corresponding peaks are 481
and 483 nm. In solid PL, similar features of Al(Saq)3 and Al(SCarq)3
were observed at 489 and 488 nm, respectively. However, it is
clearly observed that a shoulder appeared on the long-wave side
in Fig. 3d. In our previous study on Al(Saq)3 crystalloid, we found
that sizeable gaps existed between the central metal and ligands [20]. Therefore, the electron-donating group (carbazoyl) could
be attracted easier and inserted into the electron-withdrawing
core (aluminum quinolates) in Al(SCarq)3 . As a result, the emission formed from excimer on the long-wave side is depicted in
Fig. 3d. However, similar phenomena has not been observed in
both PVK/Al(Saq)3 [15] and PVK/tris(5-piperidinylsulfonamide-8quinolinolato-N1 ,O8 )aluminum [Al(QS)3 ] [14] hybrized thin films.
It presented that carbazoles pended on PVK were confined in the
poly(vinylene) backbone. Thus, the formation of exciplex was suppressed [21].
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J.-A. Cheng et al. / Materials Chemistry and Physics 113 (2009) 1003–1008
Fig. 2. AFM image of Al(SCarq)3 thin film, spin-coated from 1,2-dichloroethane.
Fig. 3. Comparison of optical properties of Al(SCarq)3 and Al(Saq)3 . UV–vis
absorption; (a) Al(SCarq)3 thin solid film, and (b) Al(Saq)3 thin solid film. Photoluminescence spectra: (c) Al(SCarq)3 in 1,2-dichloroethane; (d) Al(SCarq)3 thin solid
film, (e) Al(Saq)3 in 1,2-dichloroethane; (f) Al(Saq)3 thin solid film.
3.5. Electroluminescence
The EL behaviors were characterized by using device architectures as shown in Fig. 4. The well-known hole-injection
layer poly(3,4-ethylenedioxythiophene)/poly(styrene)-sulfonate
(PEDOT:PSS) [22] was inserted to reduce the energy barrier between the active layer and the anode. To study EL
performance, the hole-blocking layer, 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP), was added into device b.
The EL spectra of the devices are shown in Fig. 5, and their corresponding current–voltage–brightness (I–V–B) characters are also
illustrated. Identical features of EL spectra were observed in device a
and device b. However, the intensity of device b is higher than that
of device a. According to the energy diagram illustrated in Fig. 4,
the BCP layer in device b could confine the charges and excitons
within the desired regions for improved emission in these kinds
of solution-process-based devices [23]. As a result, more excitons
were generated at the active layer in device b than that of device
a. In I–V–B characterization, the turn-on voltage of device a and
device b are 9.8 and 9.6 V, respectively. Their corresponding current
efficiencies are respectively 0.25 and 0.18 cd A−1 at 5 mA cm−2 . The
maximum luminescences are 35.0 cd m−2 at 25.2 V and 19.2 cd m−2
at 26.4 V, respectively. Although the hole blocker utilized in device
b caused high current density with identical driving voltage, the
luminescence was higher than that of device a.
In addition, the broadened EL feature of both devices appeared
at 572 nm with the full width at half maximum (FWHM) of 184 nm.
Their Commission Internationale de L’Éclairage (CIE) coordinates
are (0.40, 0.47), and the chromaticity of EL is almost independent of the driving voltage. However, the EL spectra related to that
of the PL were substantially red-shifted, and it was inferred that
emission in the red region in our devices may arise from the singlet electroplex (Carbazole+ /Core− )* [24,25]. The electroplex can
be regarded as a pair of statistically independent carbazole groups
and the chromophoric core, in which one has an excess electron
(core− ), while another one has a hole (carbazole+ ). Fig. 6 schemati-
Fig. 4. Device architecture based on Al(SCarq)3 as an active layer.
J.-A. Cheng et al. / Materials Chemistry and Physics 113 (2009) 1003–1008
1007
Fig. 5. Normalized EL spectra from electroluminescent devices: (a) single layer; (b)
multilayer with BCP. The current–voltage–brightness curves of the corresponding
devices were inserted at the right corner.
cally presents the electron configuration in the molecular orbitals of
the Al(SCarq)3 electroplex. For comparison, the electron configuration of exciplex is also presented. The energy of such a hole–electron
pair trapped by the carbazole and the core should be lower than that
of a singlet excited state of a single chromophoric core. As a result,
direct cross recombination of the hole and the electron would lead
to emission at longer wavelengths compared to the fluorescence
of Al(SCarq)3 in solution. The electroplex rather than exciplex of
Fig. 6. Schematic representation for the electron configuration in the molecular
orbitals of electroplex and exciplex of Al(SCarq)3 . Path A and path B were respectively
driven by DC bias and photoexcitation.
Fig. 7. Absorption spectra of thin films: (a) Al(Saq)3 , (b) PVK, and (c) Al(SCarq)3 .
Here, (d) is the photoluminescence excitation spectrum of Al(SCarq)3 at 572 nm.
Al(SCarq)3 is favorably formed in EL devices because of the presence
of an electric field. Similar disparity in the EL and PL spectra was also
reported for other small molecules and polymer materials, where
the radiative decay of the electromer (M+ /M− )* or the electroplex
(M+ /N− )* was usually responsible for additional emission in EL. It is
notable that previously reported electromers and electroplexes in
the literature were usually formed at relatively high electric fields
[26,27], while the electroplex of Al(SCarq)3 in our experiment was
detected at a relative low electric field starting from 9.6 V across a
film thickness of 85 nm.
According to the mechanism proposed in Fig. 6, the emission
level of Al(SCarq)3 at 572 nm (around 2.2 eV) in Fig. 5 is very close
to the energy gap between the HOMO energy level of carbazoyl
groups, −5.6 eV, and the LUMO energy level of the core, −3.3 eV.
To verify the mechanism of exciplex and electroplex formation, a
series of optical spectra thin films were measured and are shown in
Fig. 7. Absorptions of Al(Saq)3 and PVK in thin solid film are shown
in Fig. 7a and b, respectively. Their absorptions appear at 381, 331,
and 344 nm, respectively. The spectrum of Al(SCarq)3 , which structurally combines carbazoyl groups and Al(Saq)3 , can be observed
not only the absorption of Al(Saq)3 at 376 nm, but also a shoulder on
the left side. This was generated from carbazoles at 332 and 346 nm,
as shown in Fig. 7c. For further exploration, excitation was also measured to trace back the exciplex energy as shown in Fig. 7d. Clearly,
the feature of this spectrum is very similar to that of Fig. 7c. In Fig. 7,
however, the intensity of the excitation is much lower than that of
the inherent absorption of Al(SCarq)3 . Evidently, partial carbazole
moieties physically interact with the core and generate weak exciplex emission in photoluminescence, as shown in Fig. 3d. This result
implies that the EL emission at 572 nm is partially contributed from
exciplex, but mostly from the emission energy coming from the
electrically induced complex emission under an applied voltage.
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J.-A. Cheng et al. / Materials Chemistry and Physics 113 (2009) 1003–1008
In other words, some exciplex emission coexists with electroplex
during the electroluminescence process. As a result, EL emission
mainly appears at 572 nm, as shown in Fig. 5.
4. Conclusion
We have developed a new class of spin-coatable metallocomplex, Al(SCarq)3 via molecular engineering. The improved EL
performance of Al(SCarq)3 was also demonstrated compared with
that of Al(Saq)3 . However, the electroplex was formed between
carbazoles and the chromophoric core under the applied electric
field, and then the EL was quenched and shifted to yellow. Based
on our study, current efficiency and luminance could be further
improved by increasing the steric hindrance of hole-transporting
moiety to suppress exciplex formation. Accordingly, the development of this type of solution-processible material would be of
interest to the OLED community for development of next generation small molecule fabrication technology.
Acknowledgements
The authors are grateful to Prof. Yu-Chie Chen, Department of
Applied Chemistry, National Chiao Tung University, Taiwan, for
matrix-assisted laser desorption/ionization (MALDI) analysis. This
work was supported by the Ministry of Education of Taiwan, Republic of China under the grant “Aim for the Top University # 97W802”
and NSC-96-2628-E009-021-MY3.
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